About 35 million years ago, an asteroid hit the ocean off the east coast of North America. Its impact was a 25-mile crater now buried beneath Chesapeake Bay, an estuary in Virginia and Maryland. In the vicinity, fires, earthquakes, falling droplets of molten glass, a puff of air and a devastating tsunami were the result of this impact.
While the resulting "Chesapeake Bay Impact Crater" is now completely buried, it was discovered in the early 1990s through scientific drilling. It is now considered the largest known impact crater in the US and the 15th largest on Earth.
As the asteroid struck, an ejection layer formed, the tectites (natural glass formed by debris from meteorite impacts) and shocked zircon crystals thrown out of the impact area. Scientists refer to this stratum as the "North American tectite stray field," which covers a region of approximately 4 million square miles, about ten times the size of Texas. Some of the waste landed, while the rest cooled off immediately after contact with seawater and then sank to the bottom of the sea.
A Research Team, Including Scientist and Lead Author Marc Biren of Arizona State University's School of Earth and Space Exploration The authors Jo-Anne Wartho, Matthijs Van Soest, and Kip Hodges have received drill samples from the Ocean Drilling Project 1073 site for the first time dated with the "uranium-thorium-helium technique".
Their research has recently been published in the international journal Meteoritics & Planetary Science .
"Determining the exact and exact age of impact events is critical to our understanding of geology," said Biren. "In recent years, for example, the scientific community has recognized the importance of impact events on Earth's geological and biological history, including the dinosaurs' 65 million year old mass extinction associated with the large Chicxulub Impact Crater."
The team studied zircon crystals in particular because they retain evidence of shock metamorphosis caused by shock pressures and high temperatures associated with impact events. The dated crystals were tiny, about the size of a human hair.
"The key to our investigation was zircon – or more precisely: zirconium silicate – crystals found in the oceanic sediments of a borehole located nearly 400 kilometers northeast of the impact site of the Atlantic Ocean," says co-author Wartho Study started as head of laboratory at the ASU mass spectrometry laboratory.
For this study, Biren worked with co-authors Wartho (now at GEOMAR Helmholtz Center for Ocean Research Kiel), Van Soest and Hodges to prepare samples for analysis and date zirconium crystals using the uranium-thorium-helium dating method , Biren then identified and processed shocked zirconium fragments for imaging and chemical analysis with an electron microprobe. "Our findings demonstrate the usefulness of the uranium-thorium-helium dating method in similar cases where shocked materials were ejected from the crater and then rapidly cooled, especially in cases where the sample size is small."
Transformations to Grainy Zircon Unveiled: Meteor Crater, Arizona
M. B. Biren et al., (U-Th) / He-zircondatation of the Chesapeake Bay distal ejection from ODP site 1073, Meteoritics & Planetary Science (2019). DOI: 10.1111 / maps.13316
Researchers study the largest impact crater in the US, which has been buried for 35 million years (2019, 13 August)
retrieved on 13th August 2019
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